The U class were originally developed as unarmed training boats to replace the WWI H-class. They were fitted with torpedo tubes to allow them to be used operationally. Small and manoeuvrable (630 tons) they were well suited to the confined waters of the North Sea and Mediterranean. They were succeeded by the very similar but partially welded V-class. Cheap, reliable and quickly produced, these were the workhorses of the wartime submarine service.